Bolts for marine diesel engine cylinder heads selected as fasteners

Cylinder head bolts are powerful bolts in diesel engines, which bear the maximum explosive force that separates the cylinder head body from the cylinder head. At the same time, under the action of bolt pre-tightening force, the air tightness of the cylinder head gasket is ensured.

Bolts are generally designed as flexible structures, and the shaft diameter is slightly smaller than the inner diameter of the thread. This structure reduces the variation of the load acting on the bolt and improves the fatigue strength of the threaded connection. Although the pre-tightening stress of the shaft part has been increased, the strength of the shaft and the threaded section tends to be equal, and the structure is more reasonable. When the thread outer diameter d≥16mm, the shaft diameter d0u003d0.8d1 (d1 represents the thread inner diameter) is more appropriate, and should not be too small.

The recommended radius of the transition between the bolt shaft and the thread is Ru003d(0.75～1.0)d, the surface roughness is generally 1.25um or 0.63um, and no tool marks are allowed to improve its fatigue strength. For other measures to improve the fatigue strength of bolts, refer to the previous article. The bolts are often fastened to the cylinder body with tight-fitting threads, that is, an interference fit with the pitch diameter of the thread. When the bolt is screwed into the cylinder, a large radial stress is generated near the thread, the screw hole is tensile stress, and the thread is compressive stress. In order to reduce this stress, sometimes a taper hole is designed at the end of the bolt, and the depth of the taper hole cannot exceed the effective length of the thread. In order to reduce the bending stress of the bolt, sometimes when the bolt is screwed in, the screwed end face is in close contact with the bottom plane of the screw hole, so as to fix it firmly.

Most strong bolts are made of alloy structural steel with high tensile strength and yield strength, such as 40Cr, 38CrSi, 35CrMo, 40MnB, etc. For light-duty and heavy-duty diesel engines, sometimes high-grade alloy structural steels such as 18CrNiW and 35CrNi3W are used .

In order to reduce the stress concentration factor in the thread, it is advisable to increase the fillet radius R of the thread root. The most suitable ratio between it and the thread pitch S is R/Su003d0.18～0.21.